This invention relates to a resin-encapsulated semiconductor device, and more particularly to one which has improved resistance to bending caused by temperature variations.
A conventional semiconductor device of the type to which the present invention pertains is illustrated in cross section in FIG. 1. A semiconductor element 1 is bonded at the center of its bottom surface to the top surface of a flat base 5 by a bonding material 6. The semiconductor element 1 and the base 5 are hermetically encapsulated in a molded resin 2. The terminal pads of the semiconductor element 1 are electrically connected to a plurality of external leads 3 which extend to the outside of the resin 2 by internal leads 4 in the form of fine gold wires. The base 5 and the external leads 3 are commonly made from a copper alloy. In FIG. 1, t1 and t3 are the thicknesses of the resin 2 above and below th semiconductor element 1, respectively, and t2 is the thickness of the semiconductor element 1 itself.
FIGS. 2a and 2b are respectively a plan view and a vertical cross-sectional view of the semiconductor element 1, the base 5, and the bonding material 6. As can be seen from these figures, the area in which bonding of the semiconductor element 1 is performed is smaller than the total area of the bottom surface of the semiconductor element 1.
It is extremely difficult to obtain a reliable bond between the semiconductor element 1 and the base 5 when the semiconductor element 1 has large dimensions. If the surface area in which bonding is performed is too small, the bond will have inadequate strength, and the semiconductor element 1 may come loose from the base 5 before encapsulating can be performed. On the other hand, if the surface area of bonding is too large, as shown in FIG. 3, the semiconductor element 1 and the base 5 may be subject to bending prior to encapsulation due to temperature variations. This bending is caused by the great difference between the coefficient of thermal expansion of the silicon which constitutes the semiconductor element 1 (approximately 3.5.times.10.sup.-6 /.degree. C. for Si) and that of the base 5 (approximately 1.7.times.10.sup.-6 /.degree. C. for a copper alloy). The stresses due to this bending can easily damage the semiconductor element 1.
Even if no bending takes place prior to encapsulation, bending often takes place due to temperature variations subsequent to encapsulation, particularly when the semiconductor element 1 is large. The coefficient of thermal expansion of the resin 2 (approximately 2-5.times.10.sup.-5 /.degree. C. for an epoxy resin) is significantly less than that of the semiconductor element 1, and as the adhesion between the resin 2 and the base 5 is normally less than that between the resin 2 and the semiconductor element 1, if t1=t3, bending of the semiconductor element 1 and the resin 2 will occur as illustrated in FIG. 1 if there are temperature variations subsequent to encapsulation, leading to damage of either the semiconductor element 1 or the resin 2.
It is conceivable to temporarily prevent the bending of the semiconductor element 1 by controlling the thickness of the resin 2 so that t3&gt;t1, but as shown in FIG. 4, even in this case, the resin 2 is subject to bending when there are temperature variations, and damage to the device can not be prevented.